1. Field of the Invention
The present invention relates to a method and apparatus for detecting the position of fluid-fluid interfaces, e.g., between liquid-liquid and gas-liquid interfaces, and, more particularly, to the effecting of such a method and apparatus by use of thermal detection means. The present invention is suitable for use in the refining of molten metals with specific application to the removal of magnesium from scrap aluminum.
2. Description of Related Art and Other Considerations
Although the impetus for conceiving the present invention is to provide process control in molten metal technologies, specifically, in a process for refining scrap aluminum, it is to be understood that the present invention is as applicable to any need for detecting the position of fluid-fluid interfaces by thermal detection means.
The removal of magnesium from scrap aluminum has been discussed in several publications, of which the following two are of particular interest to the present invention, viz., "Electrolytic Removal of Magnesium from Scrap Aluminum" JOURNAL OF METALS, Vol. 36, No.7, July 1984 pp 141-43, and "Electrolytic Demagging of Secondary Aluminum in a Prototype Furnace" AFS Transactions, Vol. 94, pp. 385-390 (1986). The following excerpt from the latter article well states the reasons and background for recovering aluminum from scrap.
"The amount of aluminum in an automobile has steadily increased from an average of 40 kg in 1976 to an average of 60 kg in 1982 due to efforts to achieve higher fuel efficiency by lowering the overall weight of the vehicle. Therefore, for a constant supply of aluminum at minimum cost, casting producers may consider increasing the use of high magnesium scrap, with large potential savings over the purchase of primary aluminum. However, to conform with specifications, the production of casting alloys such as 319 from high magnesium aluminum scrap would require the removal of magnesium in excess of 0.1 wt. %. A chlorination process is most widely used by secondary smelters for demagging casting alloys. In this process, magnesium is selectively oxidized by chlorine and removed from molten aluminum in the form of a magnesium chloride dross. While the process is reasonably efficient at high magnesium content, it may create unacceptable environmental conditions in the plant. In addition, magnesium is being lost in the form of MgCl.sub.2 dross, which being hygroscopic may pose disposal problems.
"Recognizing the need for an efficient and pollution-free demagging process, we have been developing the electrochemical process described in this paper. This process recovers magnesium in the form of salt-coated globules and apparently causes no environmental problems. The process . . . consists of covering the molten aluminum scrap with an electrolyte (a mixture of alkali and alkaline earth metal halides) and passing a current between molten aluminum acting as an anode and inert cathode dipped into the electrolyte. On applying a voltage between the electrodes, magnesium (being more reactive) dissolves first in the electrolyte from the aluminum melt, and concurrently deposits on the cathode. Because of its lower density, magnesium floats on the electrolyte and, thus, it is separated from the aluminum."
Inasmuch as the reaction vessel utilized in this demagging process contains three liquid layers comprising a top layer of magnesium, a middle layer of salt-electrolyte and a bottom layer of aluminum, operators need to monitor the levels of each layer during the addition or removal of metal. In particular, precise information about the electrolyte-metal interfaces is required to permit the removal of purified aluminum from the vessel without its being contaminated with the molten salt.
In the equipment described in the above-referenced AFS Transactions publication, the problem of aluminum removal was solved by utilizing two vertically placed drain holes, similar to holes 25 and 26 herein shown in FIG. 1. As the purified aluminum was drained from the reaction furnace into separate collection vessels, the electrolyte appeared at the upper drain hole, at which point the draining process was stopped to prevent any electrolyte from draining through the lower hole. The procedure was inconvenient to use and would be difficult to automate.